Method of drying hydrogels



Patented June 19, 1928.

UNITED s'r ras PATENT OFFl-CE.

ERNEST BAIJ JWIN 0F BALTIMORE, MARYLAND, ASSIGNOB TO THE SILICA GIL CORPORATION, OF BALTIMORE, MARYLAND, A CORPORATION OI MARYLAND.

new. or DRYING rrxnxoenns 80 Drawing.

This invention relates to a method of drying hydrogels.

In the manufacture of hard, porous silica gel or other gels of a. similar nature, such as gels of oxides of tin, aluminum, tungsten or titanium, an intermediate product 1s formed which consists of a jelly-like mass containing a large amount of water, and this intermediate product is called hydrogel. The principal feature of this invention is an improved method of removing most of the water from this hydrogel, whereby the time required for this dehydrating may be considerably shortened, the cost of the operation reduced, and the output of the p ant increased.

The gel-should have a large number of ultramicroscopic pores in order that it may act as an efliclent adsorbing agent to adsorb gases, vapors, or dissolved matter, as may be desired. As an example of the pore characteristics of properly manufactured silica gel, one gram has a. total internal volume of about 0.41 cc. In other words, if we completely fill the pores in silica gel with water, the amount of water taken up will'be approximately 41% of the initial weight of thegel. Furthermore, asufiiclent percentage of the total internal volume of silica gel is made up of pores of such small' size that the gel will adsorb water to such an extent that it will contain, at 30 C. at least 21% of its own weight of water I when in equilibrium with water vapor at a partial pressure of substantially 22 mm. of

mercury.

Several methods of preparing a hydrogel which when dried accordin to the method herein disclosed will iel roduct, are known. or example, a silica hydrogel may be made by Grahams method, which consists in adding sodium silicate so lution to an acid solution in such uantities and concentrations that there is a arge excess of acid in the mixture. This mixture is then dialyzed, whereupon after many hours the mixture in the membrane sets to a hydrogel. This hydrogel when dried as disclosed herein gives a satisfactory prodnet. This method of producing the hydrogel is not commercially practicable, however, for the reason that the time required for setting is so great and the step of dialysis entails considerable loss of material.

a satisfactory Application filed October 17, 1925. Serial No. 63,146.

I prefer to employ the method disclosed in the patent to "Walter A. Patrick, No. 1,297,- 724, dated March 18, 1919, wherein solutions of a soluble'silicate and an acid are mixed in definite quantities and concentrations, so that the mixture will set to a hydrogel withing a few hours, for example, three to five. It equal volumes of sodium silicate and hydrochloric acid solutons are to be mixed, the sodium silicate solution should have a specific gravity of from 1.15 to 1.22, referably about 1.185, and the hydroch oric acid should be about a ten per cent solution by weight, having a specific gravity of about 1.05. The commercial water glass used has a specific gravity of about 42 B. and contains about 9% Na,,() and about 29 to 30% SiO This is diluted to give a solution havin the specific gravity specified above. VVTIEII the solutions of these concentrations are mixed in equal volumes, with vigorous stirring, a clear, colloidal solution of silicic acid is first formed and no precipitation takes place, but after some time the entire solution gradually. sets to a jelly-like mass of hydrogel. Any acid or any mixture of acids may be employed in connection with the above process, although hydrochloric or sulphuric acids are preferred, and any soluble silicate or mixture of soluble silicates may be substituted for sodium silicate. All that is necessary is to regulate the concentrations and quantities so that the proper stability of the mixture is attained, or in other words, so that no precipitation will take place but the mixture will gradually set to a hydrogel. In order to accomplish this result, the mixture must be homogeneous and must be kept slightly acid in all itsv parts. This may be done either by adding the silicate solution to the acid solution with vigorous stirring, or by pouring the two solutions together uniformly while stirring vigorously. The final concentration of the acid in the resulting mixture should be from about 0.3 to 1.0 gram ions of hydrogen per liter, but a concentration offrom 0.5 to 0.6 gram ions of hydrogen per liter is preferred.

According to said patent, the hydrogel after setting is broken into small pieces and washed free from excess acid and salts. The washed hydrogel is then dried in a stream of air at 75 to 120 C. After the hydrogel isnearly dry the temperature is slowly mcreased up to 300 to 400 C. An alternate method of described in the above atent is to eatin at 75 to 120 0., as above described, and su ject the same to a vacuum, the temrature during the vacuum treatment hemg from 75 to 120 C. The final roduct is a hard, glassy, porous l whic is so stable that it may be repea y activated by strong heating even to 600 0. without substantial diminution of its 'adso tive powers. A hydrogel which when. ried by the methods above described will give a stable product having the proper re character- IStlCS, willv also give a satis actory roduct when dried accordin to the metho of the present invention. it is important to ob serve, however that unless a properly prepared hydrogel is used, the resulting product will be valueless no matter what method of drying is employed. Whether or not the manufacture has been properly conducted can be determined, in the last analysis, only by considering whether a satisfactoiX product has been obtained.

ccordin to a method of drying formerly employed, t e hydrogel is placed in trays in the form of small lumps, and the trays are placed in an oven. Heated air of gradually increasing temperature up to about 250 Fahrenheit is" circulated over the trays, -thereby drying the hydrogel. The air may be circulated in a closed path and heated by passing over steam pipes. With this process about ten hours are required to dry the hydrogel to a point where the water content is :fr(l m 5% 'to 15% of the weight of the dry ge In the improved method of dr ing according to the present invention, t e hydrogel in lump form is given a preliminary drying lasting about -four hours in stationary trays in the manner above described. This preliminary drying removes from to 90% of the water originally present in the hydro: gel and leaves theel containing water to the extent of about 6 of the weight of the dry gel. The gel is. now sufliciently strong to withstand being tumbled about without injury that would prevent obtaining the desirable pore characteristics in the final product, the preservation of which, as above inted out, is an essential object of the -mvention.

The next ste 'consists in removing the partially dried I iydrogel from the trays and placing it in a rotary cylindrical drier heated to a temperature not exceeding about 500 Fahrenheit. One passage through the ro- I tary drier, which takes about ten minutes,

is suflicient to reduce the water content to frtlun 5% to 15% of the weight of the dry tale tie material after the first a plant from two to two and one-half times, resulting in a, considerable economy in the cost of manufacture of the gel.

It is to be distinctly understood that the improved method of herein disclosed is not limited in its app icationto silica gel, althou h this has been described as a specific em odiment of the invention but may be employed to obtain other gels of similar physical structure, such as gels of oxides of tin, aluminum, tun ten or-titamum, either singly or in com ination with each other, or gels of one or more of the above oxides with silica. It is also ap licable in the manufacture of impre a gels, that is to say, gels impregnat with cata- 1 tie agents such as metals and meal oxides, or example,,nickel, iron or copper, or the oxides of these metals. Suitable methods of preparing these els\ are described in detail in Patents 1,5?7,1sr and 1,577,189.

2. The method of d' ing hydrogel which-- consists in passing air 'eated to a relatively low temperature over the stationar h drogel until the water content of they rogel is reduced to such an extent that the hydroel will withstand being'tumbled about, and

t ion heatin at a considerably higher tem-' perature while the hydrogel is being agitated until the water content is reduced to the desired amount.

3. The method of drying hydrogel which consists in heating the h drogel at a relatively low temperature w ile stationary un-. .til the water content of the hydrogel is reduced to about of the weight of the dry gel, and. then heatin at a considerably higher temperature whi e the hydrogel is being agitated until the water content 1s redulced below 15% of the weight of the dry e g 4, The method of drying hydrogel which consists in passing air heated to a-relatively low temperature over the stationary h droel until the water content of the y rogel 1S reduced to about 60% of the weight of the dry el, and then heatin at a considerably hlg er tom rature wh e thehydrog'el is bein a 'tate 'until the water content is reduce be ow-15% of the weight of the dry 5. The method of drying hydrogel which consists in heating the hydrogel at a temperature not exceeding about 250 Fahrenheit while stationar until the water content of the hydroge is reduced to such an extent that the hydrogel will withstand being tumbled about, and then heating at a higher temperature while the hydrogel is being agitated until the water content is reduced to the desired amount.

6. The method of drying hydrogel which consists in passing air heated to a temperature not exceeding about 250 Fahrenheit over the stationary hydrogel until the water content of the hydrogel is reduced to suchan extent that the hydrogel will withstand being tumbled about, and then heating at a temperature of about 500 Fahrenheit while the hydrogel is being agitated until the water content is reduced to the desired amount.

7. The method of drying hydrogel which consists in heating the hydrogel at a tem erature not exceeding about 250 Fahren eit while stationary until the water content of the hydrogel is reduced to about of the weight of the dry gel and then heating at a temperature of about 500 Fahrenheit while the hydrogel is being agitated until the water content is reduced below 15% of the weight of the dry gel.

8. The method of drying hydrogel which consists in passing air at progressively increasing temperatures up to about 250 Fahrenheit over the stationary hydrogel until the water content of the hydrogel is reduced to about 60% of the weight of the dry gel, and then heating at a temperature of about 500 Fahrenheit while the hydrogel is being agitated until the water content is reduced below 15% of the weight of the dry 9. The method of drying hydrogel which consists in heating the stationary hydrogel at a temperature not exceeding about 250 Fahrenheit for about four hours, and then heating at a temperature of about 500 Fahrenheit for about ten minutes while the hydrogel is being agitated.

10. The method of drying hydrogel which consists in passing air at progressively increasing temperatures up to about 250 Fahrenheit over the stationary hydrogel for about four hours, and then heating at a temperature of about 500 Fahrenheit for about ten giinutes while the hydrogel is being agitate 11. The method of drying hydrogel which consists in subjecting the hydrogel in lump form while stationary to a gaseous medium heated to a relatively low temperature until the water content of the hydrogel is reduced to such an extent that the hydrogel will withstand being tumbled about, and then feeding the hydrogel through a gaseous medium heated to a considerably higher temperature until the water content is reduced to the desired amount.

12. The method of drying hydrogel which consists in subjecting the hydrogel in lump form while stationary to a gaseous medium heated to a relatively low temperature until the water content of the hydrogel is reduced to about 60% of the weight of the dry gel, and then feeding the hydrogel through a gaseous medium heated to a considerably higher temperature until the water content is reduced below 15% of the weight of the dry gel.

13. The method of drying hydrogel which consists in subjecting the hydrogel in lump form while stationary to a gaseous medium heated to a temperature not exceeding about 250 Fahrenheit until the water content of the hydrogel is reduced to such an extent that the hydrogel will withstand being tumbled about, andthen feeding the hydrogel through a gaseous medium heated to a temperature of about 500 Fahrenheit until the water content is reduced to the desired amount.

14. The method of drying hydrogel which consists in subjecting the hydrogel in lump form while stationary to a gaseous medium heated to a temperature not exceeding about 250 Fahrenheit until the water content of the hydrogel is reduced to about 60% of the weight of the dry gel, and then feeding the' hydrogel through a gaseous medium heated to a temperature of about 500 Fahrenheit until the water content is reduced below 15% of the weight of the dry gel.

15. The method of drying hydrogel which consists in subjecting the hydrogel in lump form while stationary to a gaseous medium heated to a temperature not exceeding about 250 Fahrenheit for about four hours, and then feeding the hydrogel through a gaseous medium heated to a temperature of about 500 Fahrenheit for about ten minutes.

In testimony whereof I hereunto afiix my signature.

' ERNEST BALDWIN MILLER. 

